Work in progress concerns the possible role of parathormone in mediating tissue buffering of an administered mineral acid load. Our recent observations in dogs indicate that equivalent quantities of hydrogen ion administered with different anions (e.g., HCl, H2SO4, HNO3) result in marked differences in tissue buffering. Because a significantly smaller fraction of an administered hydrochloric acid load is buffered by nonbicarbonate buffers than is the case for sulfuric or nitric, the decrement in plasma bicarbonate concentration observed following HCl infusion is more than twice as great as that following equivalent infusions of the other acids. Because parathormone has recently been recognized as a potential mediator of tissue buffering following ammonium chloride administration to rats, we are presently examining the possibility that a differential stimulation of this hormone may account for the observed differences in our infusion studies. The response of nephrectomized rats to standard acid infusions in the presence and absence of intact parathyroid glands is being assessed.